1. Field of the Invention
The present invention relates generally to an apparatus and method for forming a workpiece; and more particularly, to a multistage forming apparatus that forms a sheet of material at an elevated temperature into a workpiece having a complex geometry.
2. Description of Related Art
Superplastic forming (SPF) is a process that takes advantage of a material's superplasticity or ability to undergo large strains under certain elevated temperature conditions. Superplasticity in metals is defined by very high tensile elongation and is the ability of certain materials to undergo extreme elongation at a predetermined temperature and strain rate. A simple example of superplastic forming is gas forming a sheet of material placed within a single-sided die set having a planar seal surface. The die and sheet of material, or as sometimes referred to the blank, are heated to a superplastic temperature and a predefined gas pressure profile is applied to one side of the sheet. The gas pressure forces the sheet into a die cavity and against a mold surface while maintaining a predetermined or target strain rate during the forming cycle.
Taking advantage of the superplasticity of the material enables forming of complex components not normally formed by conventional room temperature metal forming processes. For example, in the automotive industry it is often necessary to form components having deep cavities and very small radii. While superplastic forming enables forming such a component, one disadvantage is that it normally requires relatively long forming cycle times. Specifically, a conventional SPF process used to manufacture a complex part can require a forming cycle time as high as 30 minutes. Reducing these cycle times can result in necking and splitting when forming over small radii and excessive thinning of the part in certain areas such as the inside corners of concave parts or at the bottom of the mold cavity.
One way to address such problems includes prethinning the blank through a separate step prior to performing the superplastic forming process. Such a step can either include a mechanical step that draws additional material into the die cavity or a pre- or initial superplastic forming process that stretches or pre-thins a portion of the blank prior to the blank undergoing the final superplastic forming step. Depending upon the depth or configuration of the part, these additional steps typically require additional forming equipment, which increases costs, and in some instances may increases the forming time of the part.
While attempts have been made to combine a mechanical process for pre-forming the metal sheet or blank prior with a superplastic forming process such systems may require complicated pre-forming equipment that is expensive to manufacture, such as a double-action die set having a blank holder or binder assembly. Accordingly, there exists a need for a reduced complexity superplastic forming apparatus for forming metal sheet or blanks that combines the benefits of both mechanical and superplastic forming processes.